This invention relates generally to improved hygrometer apparatus for measuring atmospheric water vapor content The invention is particularly concerned with hygrometer transducer constant temperature operating apparatus and sorption hygrometer transducer apparatus that have sensitive and stable performance, and which may be fabricated simply and economically. The invention also includes constant temperature operation of refractory substrate-supported capacitance, resistance, and surface acoustic wave humidity transducers and substrate-supported transducers generally.
The art of electric hygrometry is a mature and well-developed art. Hygroscopic materials have been used as sorption detectors on crystal microbalances, as dielectrics in capacitors, and as coatings over electrodes which together operate as variable resistances. Among the more sensitive hygrometers are piezoelectri quartz crystal sorption hygrometers which work through the adsorption of water vapor onto a hygroscopic coating on an oscillating quartz crystal. The resulting increase in weight changes the frequency of oscillation by an amount proportional to the weight gain and the instrument functions as a very sensitive microbalance. Several examples of a microbalance mechanism used as a coated piezoelectric analyzer are given by U.S. Pat. No. 3,164,004 in which we are taught about various materials which may be used as sensitive coatings and we are also taught details of the microbalance technique. In 1969 William H. King, Jr. extensively described "Using Quartz Crystals as Sorption Detectors" on pages 28-34 and 28-33 of the April and May issues, respectively, of the journal "Research/Development". Microbalance technique is further taught by U.S. Pat. No. 3,253,219 which describes corrosion rate measurements using the crystal microbalance technique. Later, U.S. Pat. No. 4,562,725 teaches us about a family of moisture sensitive film coatings which may be used on microbalance crystals as well as on resistance transducers.
The use of metal oxides in moisture sensing, and aluminum oxide in particular, is taught by U.S. Pat. No. 2,237,006 which describes a capacitance type moisture sensor using aluminum oxide as a hygroscopic layer between capacitor plates. U.S. Pat. No. 3,075,385 further develops the approach using aluminum oxide as a dielectric in a capacitance hygrometer for radiosondes. U.S. Pat. Nos. 3,523,244 and 4,143,177 also describe capacitance hygrometers which use aluminum oxide as a moisture sensitive element between capacitor plates and the latter patent also teaches us the use of aluminum oxide and silicon dioxide as moisture responsive elements in semiconductor device constructions. In addition, we are taught about the use of separate heater resistors and temperature sensing resistors or semiconductors that are used in conjunction with the described oxide dielectric capacitance hygrometers.
Polymer films are widely used as moisture sensitive elements and U.S. Pat. No. 4,164,868 teaches us about the use of hygroscopic polymer films as dielectrics in capacitive humidity transducers. U.S. patents that describe and define other hygroscopic films in capacitance humidity transducers are U.S. Pat. Nos. 3,350,941; 3,582,728 and 3,802,268.
Prior art hygrometer sensors are generally operated at ambient temperature and can easily become loaded with moisture which limits their response Aluminum oxide humidity sensors often demonstrate poor calibration stability owing to ambient temperature operation. The reaction of aluminum oxide with water, as in the aluminum oxide-hydroxide reaction rate, manifests itself as calibration instability, often confused with hysteresis. Many of these same deficiencies are exhibited by capacitive hygrometer transducers which use hygroscopic film materials as a dielectric. In particular, if such transducers become wet or saturated, an extremely long time must pass until they dry off and again become responsive to atmospheric moisture change. The performance of certain capacitance hygrometer sensors is partially constrained or reduced by the presence of a necessary metal electrode on both faces of the hygroscopic layer.